Ultrafine particulate titanium dioxide has been used in a wide range of applications and, for example, as an additive to an ultraviolet-shielding material or a silicone rubber, or as a dielectric raw material or a cosmetic material (the term “titanium oxide” is widely used as a common name and therefore, in the present invention, all titanium dioxides simply referred to as titanium oxide are collectively called “titanium dioxide” or “titanium oxide”). The titanium dioxide is also used as a photocatalyst, a solar cell or the like.
As for the crystal form of titanium dioxide, three types, rutile, anatase and brookite, are available and, among these, anatase or brookite titanium dioxide are excellent in photoelectrochemical activity as compared with rutile titanium dioxide and are used in photocatalysts or solar cells.
The photocatalytic activity of titanium dioxide is utilized for the decomposition of an organic material, such as in an antimicrobial tile, a self-cleaning building material and deodorant fibers, and the mechanism thereof is understood to be as follows. The titanium dioxide absorbs ultraviolet rays to generate an electron and a hole in the inside thereof. The hole reacts with the adsorbed water of titanium dioxide to produce a hydroxy radical and by the effect of this radical, the organic material adsorbed on the titanium dioxide particle surface is decomposed into carbonic acid gas or water (Akira Fujishima, Kazuhito Hashimoto and Toshiya Watanabe, Hikari Clean Kakumei (Light Clean Revolution), pp. 143-145, CMS (1997)). That is, the conditions required for the titanium dioxide having a strong photocatalytic activity are to readily generate a hole and to allow the hole to easily reach the titanium dioxide surface. In Kazuhito Hashimoto and Akira Fujishima (compilers), Sannka Titan Hikari Shokubai no Subete (All About Titanium Oxide Photocatalyst), pp. 29-30, CMC (1998), anatase titanium dioxide, titanium dioxide with a small number of lattice defects, and titanium dioxide of a small particle size and having a large specific surface area are described as the titanium dioxides having a high photocatalytic activity.
As for the application to a solar cell, a dye-sensitized solar cell comprising a combination of titanium dioxide with a ruthenium-based dye was reported in 1991 by Graetzel et al. of EPFL-Lausanne and since this discovery, studies have been made thereon (M. Graetzel, Nature, 353, 737 (1991)). In the dye-sensitized solar cell, the titanium dioxide plays the role of a support for the dye as well as an n-type semiconductor and is used as a dye electrode bound to an electrically conducting glass electrode. The dye-sensitized solar cell has a structure that an electrolytic layer is interposed between a dye electrode and a counter electrode, where the dye absorbs light and thereby generates an electron and a hole. The electron generated penetrates through the titanium dioxide layer to reach the electrically conducting glass electrode through and is taken outside. On the other hand, the hole generated is transferred to the counter electrode through the electrolytic layer and combines with an electron supplied through the electrically conducting glass electrode. One of the factors for elevating the characteristic feature of a dye-sensitized solar cell is that the titanium dioxide and the dye are easily combined. As for the crystal form of titanium dioxide which can easily combine with the dye, for example, JP-A-10-255863 describes use of an anatase type, and JP-A-2000-340269 states that the brookite type is suitable for a dye-sensitized solar cell.
In the light of bringing out the function of titanium dioxide, good dispersibility is important. For example, when the titanium dioxide is used as a photocatalyst, if the dispersibility is bad, the covering property is intensified and the applicable usage is restricted. The titanium dioxide having poor dispersibility transmits light less and, therefore, also in the field of solar cell, the titanium dioxide capable of contributing to the light absorption is limited and the photoelectric conversion efficiency is decreased. In general, it is considered that light scattering (covering power) becomes maximum when the particle diameter is about a half of the visible light wavelength, and as the particle size becomes smaller, the light scattering is weakened (Manabu Kiyono, Sannka Titan (Titanium Oxide), p. 129, Gihodo (1991)). The primary particle diameter of the titanium dioxide used in the above-described field is from several nm to tens of nm in many cases and therefore, as long as the dispersibility is good, the effect on the light scattering is small. If the titanium dioxide has poor dispersibility and gives an aggregated particle having a large diameter, light scattering is intensified.